Casting of additionalProperties

[zoten]

Here I come again :)
But this time at least with a PR and some other questions :)

I noticed that additionalProperties seem not to be parsed properly if they are refs, when loading spec from a file.
In particular, they are not cast-ed to %Reference struct, but they remain maps, which causes later a Cast failure.
The PR addresses this issue. It was also possible to allow cast.ex to resolve maps containing $ref key, but seemed dirty, so I preferred modifying decode.
I also added a stub of yaml test, that adds an optional dependency. If this is unwanted I can of course go back to json-only.

The commit on top of the PR instead shows two failing tests that reveal a behaviour that I don't know if is wanted: when casting additional properties, keys are not converted to atoms. I think this is pretty fine, since the referenced value is correctly cast-ed, just wanted to point this out :)

Thanks, as always, for the great work! Hoping to come back with contributions for the previous issues sooner or later

[moxley]

Resolved in #360